Method for Ground-to-Satellite Laser Calibration System

ABSTRACT

The present invention comprises an approach for calibrating the sensitivity to polarization, optics degradation, spectral and stray light response functions of instruments on orbit. The concept is based on using an accurate ground-based laser system, Ground-to-Space Laser Calibration (GSLC), transmitting laser light to instrument on orbit during nighttime substantially clear-sky conditions. To minimize atmospheric contribution to the calibration uncertainty the calibration cycles should be performed in short time intervals, and all required measurements are designed to be relative. The calibration cycles involve ground operations with laser beam polarization and wavelength changes.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 13/713,033, which issued as U.S. Pat. No. 8,767,210 on Jul. 1, 2014,which claimed the benefit of and priority to U.S. Provisional PatentApplication Ser. No. 61/644,071, filed on May 8, 2012, the contents ofwhich are hereby incorporated by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

FIELD OF THE INVENTION

The present application relates to calibration of spaceborne radiometricsensors in reflected solar wavelength range, and in particular to amethod of calibrating sensors on orbit utilizing a ground-based lasersystem, pointing at and tracking an instrument on board an orbitingsatellite during nighttime with clear-sky atmospheric conditions. Thecalibration is achieved by transmitting expanded and uniform laser beamto the instrument in low Earth or geo-stationary orbit, and varying beampolarization and its wavelength within short time intervals.

BACKGROUND OF THE INVENTION

Quality of Earth Science data products based on observations fromspaceborne radiometric sensors depends on their performance and accuracyon orbit. The accuracy of measuring reflected solar radiance can beaffected by multiple factors. First, instruments with complex optics aresensitive to polarization. The response of such instruments ischaracterized before the launch, however, sensitivity to polarizationcan change on orbit significantly (e.g. Moderate Resolution ImagingSpectroradiometer (MODIS) launched on board the Terra satellite). Otherfactors are the degradation of optics, particularly in blue wavelengthrange below 500 nm (e.g. Clouds and Earth's Radiant Energy System(CERES) launched on board the Terra & Aqua satellites), and on-orbitchanges in the instrument response to stray light. None of the existingsensors has the ability to monitor all these changes in calibration onorbit.

Accurate verification of space born sensors calibration on orbit plays acrucial role in meeting mission accuracy requirements. Onboardverification systems significantly impact mission costs by increasingthe mass of instrumentation and required power. Also, onboardverification systems are not accessible for adjustment, maintenance,improvement, or repairs in the case of failure. Accordingly, a needexists for an improved calibration concept that does not suffer from thedrawbacks of known calibration systems and methods.

SUMMARY OF THE INVENTION

The present invention is a method of calibrating an optical sensor, andmore specifically, a method of calibrating an optical sensor onboard asatellite orbiting the Earth. The calibration is achieved bytransmitting expanded and uniform laser beam to the instrument in lowEarth or geo-stationary orbit, and varying beam polarization and itswavelength within short time intervals. The method is applicable to theinstruments observing the reflected solar radiance. The method includesutilization of a ground-based laser, ground-to-space laser calibration(GSLC) system, with Continuous Wave (CW) laser, to generate a lightsignal on orbit (radiance) with controlled wavelength and polarizationon the ground. The expanded beam with uniform top-hat profile, generatedby ground-based laser system is aimed at and transmitted to thesatellite, whereby entire aperture of optical sensor on the satellite isexposed to transmitted light. The optical sensor measures the intensityof incident signal on orbit while operations with beam polarization andwavelength are performed using optics on the ground.

One aspect of the method is to determine sensor sensitivity topolarization on orbit, which can be defined as instrument response tothe same light intensity with different polarization. The concept ofcalibrating sensitivity to polarization of spaceborne sensor isillustrated in FIG. 1. The physical principle is to safely expose aradiometer on orbit to 100% linearly polarized light generated by theGSLC, and to map sensor response to polarization at differentpolarization angles within a short time period (seconds). The operationsare considered when spacecraft overpasses the ground site duringnighttime with clear atmospheric conditions. While the spaceborne sensorobserves the laser signal, with its line-of-sight aligned with the laserbeam vector, the direction of linear polarization of laser light isrotated 360 degrees using beam optics on the ground. There are three keyadvantageous points used in this approach: (a) polarization parametersof laser light are not affected by a clear atmosphere even if itsintensity changes between the surface and TOA; (b) it is a relativemeasurement: the response of the spaceborne sensor should be the same atpolarization angles 0°, 180° and 360°, and this feature provides anormalization reference for the calibration cycle; and (c) themeasurements should be performed during a short time interval—withinseconds. Although the Earth's atmosphere attenuates the intensity of thelaser beam, during short time intervals (e.g. 5-10 seconds) theatmospheric conditions do not substantially change, and beam intensityshould be as stable on orbit as it is on the ground. Since thecalibration is relative, only stability of the beam is required, andtherefore, application of a Continuous Wave (CW) laser is preferablechoice.

Another aspect of the present invention is verification of instrumentspectral response. The GSLC can be used to verify sensor spectralresponse on orbit by using lasers with different wavelengths or atunable laser. In this case, the beam polarization is not required. Thespectral verification measurements are also relative—ratio of instrumentresponse in blue to its response in red and near-infrared (NIR)wavelengths as function of time from mission start date (generally,degradation in NIR is negligible). Atmospheric correction must beapplied depending on the wavelength to improve accuracy of sensorcalibration. Additionally, it is possible to map Relative SpectralResponse (RSR) of narrowband and hyperspectral instruments by changinglaser wavelength fast (e.g. 5-10 seconds) by tuning the laser around acentral wavelength. The measurement is also relative, being normalizedto the sensor response at the central wavelength.

The present invention can also be used to verify instrument response toboth kinds of stray light on orbit, geometric and out-of-band, bychoosing appropriate geometry of observations and laser wavelength.During nighttime the laser beam tracks the satellite as it comes up overthe horizon until it goes down. Meanwhile, the on-orbit sensor acquiresdata in its nominal mode. All measurements are normalized to the signalfrom direct view of the laser site by the sensor to provide a relativemeasurement. The same operation is performed for many orbit tracks inorder to develop an integrated stray light model. Verification ofout-of-band stray light, or spectral cross-talk, can achieved byoperating the laser at a selected wavelength and mapping instrumentresponse in near-by bands.

The present invention includes several unique aspects or features,including radiometric measurement of ground-based laser signal for longtime periods (e.g. 2-5 minutes) by an on orbit sensor. This requiressatellite tracking by a laser system and also requires that the sensoris pointing at the laser site on Earth surface. All calibrationmeasurements are relative, and the measurements are performed duringshort time intervals (i.e. the duration of the signal is about 5-10seconds). Atmospheric effects cancel out due to the short time interval.The polarization parameter of laser light is not affected by clearatmosphere. Aerosol can de-polarize laser beam for 0.1-0.2% at most(forward scattering), but at a high altitude site this effects arenegligible. Operations including radiometric calibration usingpolarization and wavelength operations have not previously beenperformed utilizing a ground-to-space laser.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially fragmentary view of a satellite on orbit showingan operational concept for calibrating sensor sensitivity topolarization on orbit;

FIG. 2 is a partially fragmentary view of a satellite on orbit shown anoperational principle for mapping stray-light response; and

FIG. 3 is a block diagram of a ground-based laser system for radiometriccalibration of sensor on orbit. Suggested sub-system structure reflectsmethodology of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof shall relate to the invention as oriented in FIG. 1. However, itis to be understood that the invention may assume various alternativeorientations and step sequences, except where expressly specified to thecontrary. It is also to be understood that the specific devices andprocesses illustrated in the attached drawings, and described in thefollowing specification, are simply exemplary embodiments of theinventive concepts defined in the appended claims. Hence, specificdimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting,unless the claims expressly state otherwise.

The present invention utilizes a ground-based laser system to generate acontrolled beam that is aimed at a satellite passing overhead. Theresponse of on board sensors to changes in polarization and/orwavelength is utilized to calibrate the sensors to account fordegradation of the sensors. The ground based laser system can also beutilized to measure sensor response to stray light. This information canbe utilized to develop a stray light model for calibrating the sensors.

The above operations are preferably operated at nighttime clear-skyatmospheric conditions. Clear-sky atmospheric conditions means anatmospheric transmittance level of about 80-100% (in optical windowwavelength range) and a level of turbulence (C_(N) ²) of less than orequal to about 10⁻¹³. Optimal conditions are usually found at mountaintop levels on Earth during nighttime, which is when solar reflection issubstantially eliminated. For purposes of this invention, nighttimeoccurs when the solar zenith angle is equal or greater than about 100degrees.

Ground-to-Space Laser Calibration (GSLC) System:

A monitored Continuous Wave (CW) laser system is used to enable theknowledge of the beam intensity at about 0.1%. The required power,transmitted by the Laser System, depends on the type of the targetsensor and its orbit. For most application the transmitted power up to 3Watts should be sufficient to achieve high signal-to-noise calibrationmeasurements. The system produces a fully polarized beam, and withaccurate determination of polarization angle and ability of changing thebeam within short time intervals (e.g. 5-10 seconds). Also, the laserlight profile at satellite altitude must be uniform within requiredaccuracy, so called “top-hat” beam profile. This beam profile mitigatesthe effects of atmospheric turbulence. Several technologies can beutilized to ensure sufficient uniformity of the laser beam profile. Forexample, optical diffusion techniques can be used to obtain uniformityof laser beam cross-section within a few percent RMS. The laser ismounted on a 2-dimensional elevation-over-azimuth gimbal, with theability to point and track the satellites in low Earth andgeo-stationary orbits. As described in more detail below, the lasersystem can be utilized to determine instrument sensitivity topolarization, instrument response to spectral degradation, andinstrument response to stray light on orbit by performing the operationswith beam optics on the ground.

Calibration of Sensitivity to Polarization:

Spaceborne sensors with a complex optical design are sensitive to thepolarization of incoming light in the reflected solar wavelength range:from 0.25 μm to 3 μm. Because reflected light at the top-of-atmosphere(TOA) is linearly polarized to varying degrees (depending on scene type,view geometry, and wavelength), the sensitivity to polarization is asource of radiometric uncertainty. Generally, response of theseinstruments to polarization is characterized before launch, andsensitivity can be 2%-4% for known imaging radiometers, depending onband (e.g. MODIS, VIIRS). The sensitivity to polarization may be up to10% -15% for known hyper-spectral instruments (e.g. SCIAMACHY).Sensitivity to polarization tends to change on orbit, particularly overa long operational time period as optics degrades. Currently, there isno robust approach to monitor instrument sensitivity to polarization onorbit with high accuracy.

With reference to FIG. 1, an operational concept for calibrating sensorsensitivity to polarization on orbit includes utilizing an expanded,uniform, and linearly polarized laser beam 10 is transmitted to anover-passing satellite 1 on orbit during nighttime clear atmosphereconditions, and, as radiometer onboard observes laser signal, directionof linear polarization is rotated 360° in short time intervals(seconds). The line-of-sight of target sensor is aligned with the laserbeam vector. Variable beam polarization direction is indicated with Px,and Earth is noted with 5. A satellite 1 having onboard sensor orbitsabout the Earth 5. The satellite 1 may be in a low Earth orbit (LEO) ora geostationary Earth orbit (GEO). The satellite 1 carries radiometricsensor which measures reflected solar radiance.

According to one aspect of the present invention, expanded, uniform, andlinearly polarized laser beam 10 is pointed at and transmitted to theover-passing satellite 1 during nighttime clear atmosphere conditions.The physical principle is to expose the radiometer on orbit to 100%linearly polarized light generated by a ground-based laser, and to mapsensor response to polarization at different polarization angles withinshort time intervals. As an onboard radiometer observes the lasersignal, with its line-of-sight co-aligned with laser beam vector (sensorpointing), direction of the linear polarization is rotated 360° in ashort time period using beam optics on the ground 6. As the satellite 1passes over, the on board sensor makes measurements continuously,typically from 15 to 100 per second. Time period of satellite overpassis about 2 minutes on average. During this 2 minutes the sensor “stares”at, or is in substantially direct visual communication with, the lasersite while the laser is pointing and tracking the satellite. To ensureatmospheric attenuation is cancelled, the complete calibration cycle ismade quickly. This ensures that the laser beam goes through the same oralmost the same atmosphere. Thus, it is preferable to have completerotation of the polarization direction, or switch between blue and redwavelength in 5-10 seconds. Each 5-10 second calibration cycle is anindependent calibration “event,” which is analyzed off line after alldata is merged. The product of interest is relative, the goal is toobtain ratio of sensor response to various polarization to its responseto 0, 180, and 360 deg polarization direction:

R=S(p)/S(0)

R=S(p)/S(360)

R=S(p)/S(180)

The S(0), S(180) and S(360) responses should be similar if theatmosphere was stable during calibration.

The instrument response to the same light intensity with varyingpolarization constitutes its sensitivity to polarization.

There are several key factors involved in this approach. First,polarization parameters of laser light (degree and direction of linearpolarization) are not affected by a clear atmosphere, even if itsintensity changes between the surface and TOA. This can be confirmed indetail using polarimetric data from existing spaceborne sensors (e.g.PARASOL) over clear-sky ocean scenes, and modeling results usingradiative-transfer approach. Second, the calibration measurements arerelative—the response of the spaceborne sensor should be the same atpolarizations direction 0°, 180° and 360°. This restricting conditionprovides a quantitative measure of verification accuracy andnormalization reference for entire calibration cycle. Third, althoughthe Earth's atmosphere attenuates intensity of the laser beam, duringshort time intervals (e.g. 5-10 seconds) the atmospheric conditions donot change, and beam intensity should be as stable on orbit as it is onthe ground. For relative measurements, only stability of the beamintensity is required, not the absolute radiance. The described approachshould be applicable to various wavelengths with a focus in the bluewavelength range, where the sensitivity to polarization is usually atthe largest levels.

Calibration of Optics Degradation:

The ground-based laser system can be used to verify sensor spectralresponse on orbit by using lasers with different wavelengths or atunable laser. In this case, the beam polarization is not required, andexpanded, uniform, and non-polarized beam is pointed at and transmittedto on orbit sensor during nighttime clear atmosphere conditions. As anonboard radiometer observes the laser signal, with its line-of-sightco-aligned with laser beam vector (sensor pointing), the wavelength ofthe laser beam is alternated between blue and near-IR within short timeintervals (e.g. 5-10 seconds). The spectral calibration measurementscomprise a ratio of instrument response in blue to its response innear-IR wavelengths as function of time from mission start date(generally, degradation in near-IR is negligible), and the spectralcalibration measurements are therefore relative. Atmospheric correctionmust be applied depending on the wavelength to improve accuracy ofsensor calibration. Additionally, Relative Spectral Response (RSR) ofnarrowband and hyperspecral instruments can be mapped by tuning/changingthe laser wavelength fast (seconds) around the central wavelength of theRSR, and differencing the on-orbit instrument responses. To make thecalibration relative, all measurements should be normalized to thesensor response at the central RSR wavelength.

Calibration of Response to Stray Light:

The ground-based laser system and method according to the presentinvention may also be utilized to verify the instrument response tostray light on orbit. In general, there are two different types of straylight. The first type is geometric by nature, meaning that light may notoriginate directly from the field of view of the imaged pixel. Dependingon the severity of the stray light, this can adversely affect thequality of remote sensing imagery and be difficult to correct. Thesecond type is spectral by nature, and it can occur if an instrument issensitive to out-of-band light. The system and method of the presentinvention can be applied to both kinds of stray light verification onorbit by choosing appropriate geometry of observations and the laserbeam wavelength. FIG. 2 is a schematic drawing showing use of aground-based laser system to determine variations in the effects ofgeometric stray light with respect to on board instrument. According tothis aspect of the present invention, an expanded, uniform, andnon-polarized laser beam 10 is transmitted to target sensor on an orbitat various locations (1A, 1B and 1C) during nighttime clear-skyatmosphere conditions, as satellite passes from limb to limb (fromlocations 1A to 1C). The sensor operates in its nominal data collectingmode, e.g. cross-track scanning. Ali stray light measurements should benormalized to the direct view of the laser site.

For this verification operation, it is preferable that laser beam is notpolarized. Therefore, during nighttime and clear-sky atmosphereconditions expanded, uniform, and not-polarized laser beam is pointed atand transmitted to the satellite 1 as it comes up over horizon, movingfrom location “1A” to location “1C.” During this time, the on orbitsensor acquires data, operating in nominal data collecting mode (e.g.cross-track scanning). To make this calibration approach relative, allmeasurements are normalized to the sensor direct view of the laser site.The operation is performed for many orbit tracks in order to develop anintegrated empirical stray light model comprising the normalizedmeasurements at a plurality of angles relative to the direct vieworientation. This setup is comparable to having “controllable” starswith adjustable view geometry and intensity. Using coincidentmeteorological data, theoretical atmospheric corrections can be used toimprove the accuracy of the derived stray light model.

The calibration of out-of-band stray light, or spectral cross-talk, canachieved by operating the laser at certain wavelength and mappinginstrument response in the near-by bands. In this case the line of-sightof laser beam and sensor must be co-aligned.

Diagram of Ground-to-Space Laser Calibration (GSLC) System:

With further reference to FIG. 3, the radiometric sensor on satellite 1comprises an on-orbit target at which laser beam 10 is directed from atransmitting telescope 12. During calibration sequence, as the satellite1 over-passes the GSLC site, the target sensor receives input fromsensor pointing operations 4 and points at the laser ground location, sothat sensor line-of-sight is co-aligned with laser beam vector. Outputof the target sensor measurement is raw Level-0 data 7, which isutilized to generate radiometric Level-1B data product 8. Using accuratetime record, the sensor Level-1B data is merged with laser systemdiagnostic, monitoring and auxiliary data (28, 30, 32, 34, 44), and withcoincident meteorological parameters provided by a weather station.

Tracking mechanics 26 is used to point the transmitting telescope 12 andlaser beam 10 to ensure it is incident on the satellite 1. The beam isgenerated by a Continuous Wave (CW) tunable laser 38, formed into therequired uniform beam profile and polarization by transmit optics 40,and then expanded by beam divergence control optics 42. The beampreferably has a uniform “top hat” profile with a RMS of about 1-2% overthe central part of the beam. The beam is preferably expanded, with adivergence of about 7-10 arcseconds. The laser diagnostic systemincludes a beam intensity monitor 28, a beam polarization monitor 30, abeam profile monitor 32, and sensors providing auxiliary data 34. Asafety shutdown feature 44 included for ability to turn off CW laser 38power according to a predefined criteria of operations (e.g. approachingaircraft). Part of laser beam transmitted to orbit 10 is reflected backto ground by the satellite 16. A receiving telescope 14 receivesreflected laser signal from satellite 16 and provides data to a PhotoMultiplying Detector (PMD) 13. The reflected signal is utilized todetermine received light intensity 22 and polarization 24. The receivingtelescope 14 is also used for calibration of tracking mechanics (starcalibration method, 18). Data from the beam intensity, beampolarization, and beam profile monitors 28, 30, and 32 respectively, andauxiliary data 34 are gathered at data acquisition element 36, and thensupplied to data merging element 20. The merged data is then analyzedoffline at 46. An auxiliary weather station 56 provides meteorologicalparameters to the data acquisition element 36. The GSLC operations 48are implemented via the on-site GSLC software 50. The GSLC system islocated at a hosting site 52 having a hosting dome 54 with azimuthtracking. The dome 54 is preferably an elevated dome that is about 20feet from the ground to avoid a thermal boundary layer.

The present invention has several advantages over space-based (onboardsatellite) calibration/verification systems. Because it is aground-based implementation, it is accessible for adjustments,development, maintenance, etc. Its calibration traceability to NISTstandards can be straight-forward in comparison to onboard calibrationdevices. Operations according to the present invention are notrestricted by on-orbit issues (SIC, fuel, etc.), the GSLC system can beused for calibration of multiple spaceborne sensors: includinghyperspectral and narrowband imagers, and polarimetric instruments.

Geolocation of the laser system is an important factor. The highaltitude sites of large optical telescope arrays are very appropriate.For optical telescopes, most ground-based observatories are located farfrom major centers of population to avoid the effects of lightpollution. The ideal locations for modem observatories are sites thathave dark skies, a large percentage of clear nights per year, dry air,and are at high elevations. At high elevations, the Earth's atmosphereis thinner, water vapor is low, the atmospheric turbulence is reduced,and thereby minimizing the effects of atmospheric conditions andresulting in better astronomical “seeing” and smaller laser beamwandering. Sites that meet the above criteria for modem observatoriesinclude, but not limited to, the southwestern United States, Hawaii,Canary islands, the Andes, and high mountains in Mexico such as SierraNegra. Major optical observatory sites include Mauna Kea Observator (BigIsland of Hawaii), Kitt Peak National Observatory (Arizona-SonoranDesert), Fred Lawrence Whipple Observatory (Mount Hopking, Ariz.) in theUSA, Roque de los Muchachos Observatory in Spain, and ParanalObservatory in Chile. These sites also have existing infrastructure suchas meteorological and computing facilities, network availability, etc.

For obtaining comprehensive information about the calibrationmeasurements, laser monitoring and diagnostics data may be recorded withnecessary frequency (beam intensity, polarization, beam divergence, andbeam profile). Recording rate up to 1 kHz is appropriate for Earthobserving sensors, and it can be accomplished easily with available dataacquisition technology. Meteorological data should be recorded at thetime of calibration measurement and operations. Such data is availableat large optical telescope sites (or generated by auxiliary weatherstation), and it enables necessary corrections for off-line dataanalysis. Also, uncertainty contribution from stray light are preferablyminimized (e.g. Moon location.) Also, as the satellite is exposed to thelaser light, the amplitude of the reflected light can be measuredaccurately and frequently on the ground, and its variation during thecalibration process can be estimated. Operations with large variationscan be rejected as not meeting required stability. Although thereflective property of a spacecraft can change long-term, it is assumedto be constant during the calibration process of a few seconds.

What is claimed is:
 1. A method of calibrating an optical sensor onboard a satellite orbiting the earth, the method comprising: utilizingat least one ground-based laser to generate a first light signal havinga first wavelength, and a second light signal having a second wavelengththat is not equal to the first wavelength; pointing the optical sensorat the one ground-based laser; aiming the first and second light signalsat the satellite whereby at least a portion of the first and secondlight signals are incident on an optical sensor on the satellite;measuring the response of the optical sensor to the first and secondlight signals; utilizing a difference between the response of theoptical sensor to the first and second light signals to calibrate theoptical sensor.
 2. The method of claim 1, wherein: the first lightsignal comprises red light, and the second light signal comprises bluelight.
 3. The method of claim 2, wherein: the first light signal isgenerated by a first laser, and the second light signal is generated bya second laser.
 4. The method of claim 1, wherein utilizing a differencebetween the response of the optical sensor further comprises: applyingatmospheric correction to improve the accuracy of the optical sensor. 5.The method of claim 1, wherein said first and second light signals areaimed at the satellite during nighttime clear atmospheric conditions. 6.The method of claim 1, wherein said first and second light signals areaimed at the satellite during clear-sky atmospheric conditionscomprising an atmospheric transmittance level of about 80-100% in theoptical window wavelength range and a level of turbulence of less thanor equal to about 10⁻¹³.
 7. The method of claim 6, wherein said firstand second light signals are aimed at the satellite when a solar zenithangle is equal or greater than about 100 degrees.
 8. A method ofcalibrating an optical sensor on board a satellite, the methodcomprising: utilizing at least one ground-based laser to generate afirst light signal having a first wavelength, and a second light signalhaving a second wavelength that is not equal to the first wavelength;pointing the optical sensor at the one ground-based laser; aiming thelight signal at the satellite whereby at least a portion of the lightsignal is incident on an optical sensor on the satellite; measuring theresponse of the optical sensor to the light signal; utilizing adifference between the response of the optical sensor to the first andsecond wavelengths to calibrate the optical sensor.
 9. The method ofclaim 9, wherein: the first light signal is generated by a first laser;and the second light signal is generated by a second laser.
 10. Themethod of claim 9, wherein the at least one ground-based laser is onelaser.
 11. The method of claim 10, wherein aiming the first and secondlight signals at the satellite further comprises: alternating betweenthe first and second wavelengths within short time intervals.
 12. Themethod of claim 11, where the short time intervals comprises 5-10seconds.
 13. The method of claim 11, wherein utilizing a differencebetween the response of the optical sensor further comprises: applyingatmospheric correction to improve the accuracy of the optical sensor.14. The method of claim 11, wherein said first and second light signalsare aimed at the satellite during nighttime clear atmosphericconditions.
 15. The method of claim 14, wherein said nighttime clearatmospheric conditions comprise an atmospheric transmittance level ofabout 80-100% in the optical window wavelength range, a level ofturbulence of less than or equal to about 10⁻¹³, and a solar zenithangle equal or greater than about 100 degrees.
 16. A method ofcalibrating an optical sensor on board a satellite, the methodcomprising: utilizing at least one ground-based laser to generate afirst light signal having a first wavelength, and a second light signalhaving a second wavelength that is not equal to the first wavelength;pointing the optical sensor at the one ground-based laser; aiming thefirst and second light signals at the satellite whereby at least aportion of the first and second light signals are incident on an opticalsensor on the satellite, and wherein an atmospheric transmittance levelcomprises about 80-100% in the optical window wavelength range, aturbulence level comprises less than or equal to about 10⁻¹³, and asolar zenith angle comprises at least about 100 degrees; measuring theresponse of the optical sensor to the first and second light signals;utilizing a difference between the response of the optical sensor to thefirst and second light signals to calibrate the optical sensor; andapplying atmospheric correction to improve the accuracy of the opticalsensor.